4) A pair of fair dice is tossed. Find the probability that at least one die shows a 3.

5) A red bag contains 4 black and 3 white marbles. A yellow bag contains 2 black and 5 white marbles. A fair coin is tossed. If the coin turns up heads, a single marble is taken from the red box. If it turns up tails, single marble is taken from the yellow box. Find the probability that the marble taken is black.

6) A fair coin is tossed four times. What is the probability of having precisely two heads?